The active switching elements of integrated circuits are interconnected by metal lines deposited by various methods such as physical vapor deposition, chemical vapor deposition, and evaporation. Typically, several levels of metal lines are used in an integrated circuit to allow crossovers. At certain locations, electrical contact is made between lines at different levels. Such contacts are called vias. The drive of integrated circuits to submicron geometry has resulted in vias and contacts of extreme aspect ratio and size. Filling submicron vias and contacts with a conducting metal is extremely difficult. If extreme care is not taken, voids will remain and the wafer must be discarded.
Aluminum thin film metallization is the primary material for conductor of metallization on large scale integrated (LSI) and very large scale integrated (VLSI) circuits. Manufacturers of these devices require the interlevel contact of a metal layer and the silicon layer beneath a dielectric. In addition, interconnects between metal layers separated by a dielectric may be necessary. To reach this end, the conductor metallization must cover vertical or near vertical surfaces in addition to the circuit areas of the substrate. Providing adequate material within the interconnect measured as step coverage, can become difficult depending on the aspect ratio, vertical wall angle, and thin film deposition technique. Physical vapor deposition primarily as evaporation or sputtering, is a method must commonly employed for aluminum deposition. The drawback of these line of site deposition techniques is the self-shadowing effect when any appreciable amount of material has been deposited. The self-shadowing limits step coverage and introduces defects into the interconnect metallization.
Various methods have been proposed for filling interconnecting vias and contacts with aluminum. Surface diffusion is used to move sputter deposited aluminum into vias at temperatures of approximately 450.degree. C. This is disclosed for example in Armstrong et al U.S. Pat. No. 4,994,162. This uses a low temperature seed layer providing a continuous high quality diffusion path for subsequently deposited material to diffuse along. A high temperature, low deposition rate step to allow efficient surface diffusion into the feature is employed followed by a high temperature, high deposition rate step to complete the deposition. Bulk diffusion as a mechanism for via filling has also been proposed by Sugano et al in 1992 VMIC Conference Proceedings "Quarter micron whole filling with SiN sidewalls by aluminum high temperature sputtering." With the Sugano process, the driving fierce for via filling is the interface between a titanium surface layer and the deposited aluminum. The process requires the presence of a continuous and high quality titanium surface on the sidewall of a via.
Further, Tracy U.S. Pat. No. 4,970,176 discloses a deposition of a relatively thick layer of aluminum at a first temperature and a subsequent deposition of a thin layer of aluminum at a high temperature. The specification indicates that the temperature increase acts to reflow the aluminum through grain growth and recrystallization.
In addition to elevated substrate temperature, other techniques available to improve aluminum step coverage include bias deposition, collimation, and chemical vapor deposition. Each technique necessitates a compromise in processing and final film properties. Higher substrate temperatures and bias provide the driving force for aluminum mobility through thermal activation and physical resputtering respectively. However, film properties such as grain size, reflectivity, alloy precipitation and resistivity are significantly altered.
The use of barrier layers between the aluminum and silicon may also be necessary to limit the aluminum silicon reaction at temperatures above 450.degree. C. The use of titanium or titanium nitride layers underneath the aluminum will enhance surface mobility of the aluminum and elevated substrate temperatures. Any significant reaction between the aluminum and titanium, however, will significantly raise the film resistivity.
Collimation of the sputtered aluminum will reduce the deposition rate to 10-20% of the noncollimated and provide significant step coverage improvements at the bottom of the interconnect only. Finally, chemical vapor deposition of aluminum films free of contamination with the necessary silicon and copper dopants is very difficult.